Friction under high normal pressures

Abstract

Torsion tests ∗∗ ∗∗ Thesis — Estudio Experimental del Coeficiente de Rose a altas Presiones, by R. Lauterbach, Universidad Catolica de Chile,in partial satisfaction of the degree of Ingeniero Civil, 1966. of totally enclosed test pieces of electrolytic copper and commercially pure aluminum were made by a rotating hardened punch whose flat end rotated under load against the constrained surface of the softer metal. This method essentially prevented plastic deformation of the test piece, except under certain conditions of load at or below the test surface. Static coefficients of friction as function of average normal pressure were obtained from the load and angle of twist signals transmitted to the two legs of a Variplotter, thus giving a load-twist diagram for each test. It was found that the theory of sublayer flow agreed with the experimental results for wire-brushed surfaces of both electrolytic copper and commercially pure aluminum, but not for copper surfaces finished with grade 180 abrasive paper. This failure is believed to be due to the state of stress in the softer of the two sliders in the metal between two adjacent junctions near its surface. This combined state of stress is unknown if the closely spaced junctions are the result of surface preparation with 180 abrasive paper. In this case the assumed simple compression in determining the flow condition by the von Mises flow rule (equivalent to a critical effective stress) must be modified to include the actual combined state of stress. For lubricated surfaces the values of torque and coefficients of friction were below the theoretical values for sublayer flow at pressures more than twice the original yield strength. The results thus indicate that the condition for sublayer flow permits the prediction of a limiting friction force and coefficient of friction, providing the state of work hardening in the test surface can be determined.